This is a study of cytogenetic and cell kinetics effects of acute stress. Stress has been implicated as a factor in the cause and progression of many illnesses, not only those recognized as psychosomatic, but also in others usually identified as organic in origin. Although several physiological mechanisms have been hypothesized by which stress may act via cortico-hypothalamic pathways, and influence endocrinological and immunological processes, little attention has been focussed on how stress may induce pathological changes on a cellular and cytogenetic level. We have developed an in vivo bone marrow technique for examination of Sister Chromatid Exchanges (SCEs) and cell cycle kinetics, and examined these parameters in rats exposed to an acute stress. One group of rats was subjected to a cold water swim (CWS), another to a warm water swim (WWS) and a third to no explicit stress (NS). Both the CWS and WWS groups showed a doubling of SCE level (4.9 and 4.7 SCEs/metaphase). This was highly significant at the P less than .0001 level. In addition, the cell cycle was lengthened in both stressed groups compared with the NS group, and this difference was highly significant at the P less than .0001 level. There is also one report in the literature that mice subjected to an "open field" test, with flashes, showed a high level of bone marrow cells with chromosome aberrations 24 hours later (Seredenin, Dernev, and Vedernikov, 1980). We wish to followup these studies in the following four experiments: (1) replicate the pilot experiment; (2) control for the effect of light upon the induction of SCEs by CWS (3) test the generality of the link between stress and SCEs by examining a select group of physical and "neurogenic" stressors (i.e., footshock, noise, and CWS), for their SCE-inducing properties; and (4) determine whether the frequency of SCEs is related to the degree of stress by subjecting groups of rats to different durations of CWS. If our original finding is confirmed and extended in these experiments, then in future experiments we would plan to: 1) differentiate from among the basic properties of stress those that affect chromosomal and DNA damage, and 2) examine the physiological and cytogenetic mechanism(s) by which stress induces SCEs and alters cell cycle kinetics.